LRP6 in mesenchymal stem cells is required for bone formation during bone growth and bone remodeling.

Li C, Williams BO, Cao X, Wan M - Bone Res (2014)

Bottom Line:
Lipoprotein receptor-related protein 6 (LRP6) plays a critical role in skeletal development and homeostasis in adults.However, the role of LRP6 in mesenchymal stem cells (MSCs), skeletal stem cells that give rise to osteoblastic lineage, is unknown.Osterix(+) osteoprogenitors and osteocalcin(+) osteoblasts significantly reduced at the secondary spongiosa area, but only moderately decreased at the primary spongiosa area in mutant mice.

ABSTRACTLipoprotein receptor-related protein 6 (LRP6) plays a critical role in skeletal development and homeostasis in adults. However, the role of LRP6 in mesenchymal stem cells (MSCs), skeletal stem cells that give rise to osteoblastic lineage, is unknown. In this study, we generated mice lacking LRP6 expression specifically in nestin(+) MSCs by crossing nestin-Cre mice with LRP6(flox) mice and investigated the functional changes of bone marrow MSCs and skeletal alterations. Mice with LRP6 deletion in nestin(+) cells demonstrated reductions in body weight and body length at 1 and 3 months of age. Bone architecture measured by microCT (µCT) showed a significant reduction in bone mass in both trabecular and cortical bone of homozygous and heterozygous LRP6 mutant mice. A dramatic reduction in the numbers of osteoblasts but much less significant reduction in the numbers of osteoclasts was observed in the mutant mice. Osterix(+) osteoprogenitors and osteocalcin(+) osteoblasts significantly reduced at the secondary spongiosa area, but only moderately decreased at the primary spongiosa area in mutant mice. Bone marrow MSCs from the mutant mice showed decreased colony forming, cell viability and cell proliferation. Thus, LRP6 in bone marrow MSCs is essential for their survival and proliferation, and therefore, is a key positive regulator for bone formation during skeletal growth and remodeling.

Mentions:
To examine the bone phenotypes in the mutant mice, we performed a longitudinal analysis of bone architecture using µCT. Both male Lrp6+/− and Lrp6−/− mice showed reduced trabecular bone volume, thickness and number, and increased trabecular space at 3 months of age compared to WT littermates, and the reduction was more pronounced in Lrp6−/− mice (Figure 2). Notably, BV/TV reduced >50% (Figure 2b) and the trabecular number reduced >40% (Figure 2d) in Lrp−/− mice relative to WT mice. Female Lrp6+/− mice and Lrp6−/− mice exhibit similar changes in all these parameters compared to female WT littermates (Figure 2f–2j). Cortical bone architecture in the mutant mice of both sexes was also altered significantly (Figure 3). Cortical tissue area was reduced more than 20% in male (Figure 3b) and 15% in female mutant mice (Figure 3f) relative to controls. Cortical bone thickness was reduced by approximately 10% in both sexes (Figure 3d and 3h). The results suggest that LRP6 deficiency in nestin+ cells results in low bone mass in both trabecular and cortical bone during postnatal bone growth and bone remodeling in adults.

Mentions:
To examine the bone phenotypes in the mutant mice, we performed a longitudinal analysis of bone architecture using µCT. Both male Lrp6+/− and Lrp6−/− mice showed reduced trabecular bone volume, thickness and number, and increased trabecular space at 3 months of age compared to WT littermates, and the reduction was more pronounced in Lrp6−/− mice (Figure 2). Notably, BV/TV reduced >50% (Figure 2b) and the trabecular number reduced >40% (Figure 2d) in Lrp−/− mice relative to WT mice. Female Lrp6+/− mice and Lrp6−/− mice exhibit similar changes in all these parameters compared to female WT littermates (Figure 2f–2j). Cortical bone architecture in the mutant mice of both sexes was also altered significantly (Figure 3). Cortical tissue area was reduced more than 20% in male (Figure 3b) and 15% in female mutant mice (Figure 3f) relative to controls. Cortical bone thickness was reduced by approximately 10% in both sexes (Figure 3d and 3h). The results suggest that LRP6 deficiency in nestin+ cells results in low bone mass in both trabecular and cortical bone during postnatal bone growth and bone remodeling in adults.

Bottom Line:
Lipoprotein receptor-related protein 6 (LRP6) plays a critical role in skeletal development and homeostasis in adults.However, the role of LRP6 in mesenchymal stem cells (MSCs), skeletal stem cells that give rise to osteoblastic lineage, is unknown.Osterix(+) osteoprogenitors and osteocalcin(+) osteoblasts significantly reduced at the secondary spongiosa area, but only moderately decreased at the primary spongiosa area in mutant mice.

ABSTRACTLipoprotein receptor-related protein 6 (LRP6) plays a critical role in skeletal development and homeostasis in adults. However, the role of LRP6 in mesenchymal stem cells (MSCs), skeletal stem cells that give rise to osteoblastic lineage, is unknown. In this study, we generated mice lacking LRP6 expression specifically in nestin(+) MSCs by crossing nestin-Cre mice with LRP6(flox) mice and investigated the functional changes of bone marrow MSCs and skeletal alterations. Mice with LRP6 deletion in nestin(+) cells demonstrated reductions in body weight and body length at 1 and 3 months of age. Bone architecture measured by microCT (µCT) showed a significant reduction in bone mass in both trabecular and cortical bone of homozygous and heterozygous LRP6 mutant mice. A dramatic reduction in the numbers of osteoblasts but much less significant reduction in the numbers of osteoclasts was observed in the mutant mice. Osterix(+) osteoprogenitors and osteocalcin(+) osteoblasts significantly reduced at the secondary spongiosa area, but only moderately decreased at the primary spongiosa area in mutant mice. Bone marrow MSCs from the mutant mice showed decreased colony forming, cell viability and cell proliferation. Thus, LRP6 in bone marrow MSCs is essential for their survival and proliferation, and therefore, is a key positive regulator for bone formation during skeletal growth and remodeling.